1. Field of the Invention
The present invention relates to slide forming flexible metal leads bonded to the top of an integrated circuit chip, and more particularly to slide forming tape-automated-bonding leads without damaging a support ring secured to the tops of the leads between the chip and the lead ends while minimizing footprint size.
2. Description of Related Art
In the manufacture of integrated circuits, silicon wafers containing many individual integrated circuits are fabricated and then the wafers are cut into individual integrated circuit chips or die. The chips are subsequently packaged and interconnected to other electrical components. During packaging, electrically conductive metal leads can be bonded between a chip and a high density copper/polyimide interconnect substrate to provide proper electrical interconnection between different chips.
One technique for providing conductive interconnecting metal leads is tape-automated-bonding (TAB). The leads can be fabricated on a continuous carrier film in which the tape is a laminate of copper and a plastic insulator and the leads are etched in the copper while on the plastic carrier using well known etching processes. The film can then be perforated along the edges with sprocket holes for use in advancement and alignment of the carrier film. Individual chips can be electrically bonded to individual lead frames in the tape and the lead frames can be removed or excised from the carrier film. Usually it is necessary to bend or form the leads prior to bonding the outer lead ends to a substrate.
Each generation of integrated circuits is becoming more complex and is requiring more leads on ever decreasing lead spacings (i.e., lead pitches). The pads on the integrated circuit chips, to which the leads are bonded, are configured and spaced often for the purpose of optimizing the performance of the chip and making the best use of the available space on the chip. The result is that the lead pitch on the portions of the leads in close proximity to the chip pads is seldom uniform, and is seldom as large as the lead pitch on the portions of the leads that lay furthers from the chip. That is, the leads "fan out" between the chip and the substrate, and in the absence of a support ring (such as polyimide film), conventional lead forming methods would be likely to displace the leads in pitch as a form punch moved across those areas on the leads which are not parallel to the direction of the form punch travel. A support ring may also be necessary to keep the leads in place for assembly and test operations prior to boding the outer lead ends to a substrate. The amount of lead pitch displacement will depend on the lead dimensions and materials. For example, copper TAB leads 0.006 inches wide may not yield significant pitch displacement for relatively short leads, e.g. less than 0.040 inches long. But for relatively long leads, e.g. greater than 0.080 inches long, the pitch displacement is likely to be significant and result in alignment problems with bonding pads on a substrate.
Furthermore, it is desirable to minimize the "footprint" size of the leads, which is defined as the distance that the outer ends of the leads extend beyond the periphery of the chip in the direction parallel to the top surface of the chip after the leads are bent or formed and thus shaped to be bonded to a substrate. The advantages which flow from minimizing the footprint size of the leads include increased electrical switching speed, increased reliability, and reduced substrate size, cost and weight
Unfortunately, in the prior art the use of a support ring attached to the leads can increase the size of the footprint by at least the size of the support ring since, after applying conventional lead forming techniques, the support ring is positioned on a portion of the leads that extends away from the outer periphery of the chip and lays parallel to the top surface of the chip. One solution suggested in the prior art is to strip the support ring during the lead forming operation. This, however, may be unacceptable in certain applications.